1. Field of the Invention
This invention relates to a fingerprint detection apparatus which reads and converts a finger print into an electric signal and outputs the electric signal, and more particularly to a fingerprint detection apparatus which reads a fingerprint pattern based on a pressure distribution obtained when a finger is pressed against the fingerprint detection apparatus.
2. Description of the Related Art
As the information-oriented society and card-dependent society has develop rapidly, the strong demand for security is increasing. In order to satisfy this demand, progress is being made in development of various techniques which use a characteristic of the body of an individual to identify the individual. A technique for identifying individuals which uses a fingerprint and can be carried out readily is expected to have high applicability in the future for various terminal equipments and is expected to be put into practical use.
Most of the fingerprint detection apparatus which have been practical and widely used rely upon an optical system which employs a rectangular prism, as disclosed, for example, in Japanese Patent Laid-Open No. 13446/1980.
However, a fingerprint detection apparatus which relies upon an optical system which employs a rectangular prism is limited in miniaturization of the apparatus because the prism itself has a large size and it is difficult to make the focal length of an optical lens short. Further, since optical parts are used, reduction in cost of the fingerprint detection apparatus is also limited.
Further, the fingerprint detection apparatus which relies upon an optical system has a problem in that, when a finger is too wet or too dry, the fingerprint pattern detected is obliterated or becomes blurred. Therefore, the fingerprint detection apparatus is not sufficiently high in practical use in terms of reliably identifying an individual with certainty under any circumstances.
In order to raise reliability, a fingerprint detection apparatus is proposed in Japanese Patent Laid-Open No. 98754/1995 which does not have a structure wherein a finger is pressed directly against a surface of a prism but has a structure wherein a resilient transparent optical member and a liquid coupling member are interposed between an optical reference surface on which a finger is pressed and a surface of a prism. This structure provides closer contact between the finger and the optical reference surface and allows a stabilized fingerprint pattern to be obtained.
However, with the fingerprint detection apparatus disclosed in Japanese Patent Laid-Open No. 98754/1995 also, the optical principle on the optical reference surface is the same as that when the finger is pressed directly against the prism surface. Therefore, while the fingerprint detection apparatus achieves a little improvement in regard to the problems of reliability as described above, it is still disadvantageous in that, if the finger is sufficiently wet, the fingerprint pattern detected is obliterated, or if the surface on which a finger is pressed is soiled or foreign articles are stuck to the surface, the fingerprint pattern detected is disordered.
As described above it is particularly difficult for fingerprint detection apparatus which rely upon an optical system, to satisfy the demand for miniaturization. Therefore, fingerprint detection apparatus which do not rely upon an optical system have been proposed.
For example, a fingerprint sensor is disclosed in Japanese Patent Laid-Open NO. 27277/1983 wherein resistance elements or piezoelectric elements which have a pressure dependency are used to convert a pressure pattern obtained from concave and convex patterns of a fingerprint when a finger is pressed into a voltage pattern so that the pressure pattern is outputted as an electric signal. The document mentioned further discloses another fingerprint sensor wherein resistance elements or pyro-electric elements which have a temperature dependency are used to convert a temperature pattern obtained from concave and convex patterns of a fingerprint into a voltage pattern so that the temperature pattern is outputted as an electric signal.
The sensors disclosed in Japanese Patent Laid-Open No. 27277/1983 are formed by using an integrated circuit technique which uses silicon crystal. Conversion elements which generate voltage signals from pressures or temperatures are arranged in arrays aligned horizontally and vertically and are covered with a protective film such as an oxide film or a nitride film so that reliability will not be damaged even if a finger touches them directly.
However, such protective films are liable to be broken since they are hard and thin. Therefore, if even a small crack or pinhole is produced in the protective film, then an impurity such as sodium which sticks to the protective film by contact of a finger or the like penetrates through the crack or pinhole and becomes a cause of deterioration of the characteristics of a circuit elements such as a conversion element or a transistor.
As another conventional example of a non-optical fingerprint detection apparatus, a fingerprint sensor which detects the concave and convex configuration of a fingerprint as variations in electrostatic capacity and output the variations as electric signals is disclosed in Japanese Patent Laid-Open No. 231803/1992 and Japanese Patent Laid-Open No. 305832/1996.
In both of the fingerprint detection sensors disclosed, detection elements having detection electrodes covered with an insulating material are arranged in arrays in the row direction and column direction.
Further, the fingerprint detection sensors make use of the principle that, when a finger is pressed against a surface of the insulating material which covers over the detection electrodes, a ridge portion of the fingerprint contacts directly with the insulating material while at a valley portion of the fingerprint an air layer remains between the insulating material and the skin of the finger. Due to this principle, the electrostatic capacity between the surface of the finger and a detection electrode exhibits a higher value at the ridge portion than at the valley portion. An electric signal of a current or voltage whose variated by such a difference in electrostatic capacity as just described is outputted to detect a fingerprint pattern. Also the sensor described above are formed on a chip using an integrated circuit technique which uses silicon crystal similar to the conventional examples described above.
FIG. 1 is a diagrammatic view showing a construction of a fingerprint inputting apparatus disclosed in Japanese Patent Laid-Open No. 305832/2996.
As shown in FIG. 1, the present conventional example makes use of the fact that the electrostatic capacity is different in concave and convex portions of a configuration formed by valley lines 518 and ridgelines 519 of a fingerprint pattern against main surface 511 to which a fingerprint is brought near or contacted, and electrically detects the electrostatic capacities in accordance with the concave and convex configuration by detection circuits 513 by using electrodes 512 arranged on main surface 511 at a pitch finer than the line width of the fingerprint.
It is important that the coating of an electric material function also as a protective film for an integrated circuit, but since a protective film of a conventional integrated circuit technique is not formed to cope with the contact of a finger and besides is hard, thin and liable to be damaged, there is a problem in that the characteristics of the integrated circuit is deteriorated by mechanical damage, pollution of the integrated circuit by an impurity, and so forth.
Further, if the surface of the insulating material is soiled with sweat of a finger or the like and the insulation characteristic of the surface is deteriorated, then leak current flows along the surface and decreases the difference in capacitance between a ridge portion and a valley portion of the fingerprint. This gives rise to a problem that the contrast of the fingerprint pattern is reduced and, in an extreme case, reduced to such a degree that the fingerprint pattern itself cannot be discerned.
Another conventional example of a fingerprint sensor of the monolithic type which uses silicon crystal is disclosed in Japanese Patent Laid-Open No. 126918/1997.
In the conventional example just mentioned, a pressure pattern originating from the convex and concave configuration of a fingerprint when a finger is pressed against the fingerprint sensor is detected by pressure sensors arranged in a matrix array and is outputted as electric signals. For the pressure sensors, piezoelectric resistors, a variable capacitor and a micro-conductor positioned on an insulating layer extending above a cavity are used. Further, in the present conventional example, in order to prevent a finger from touching the protective film formed on the sensors directly, a flexible layer formed from silicone or the like is provided additionally on the protective film in order to protect the sensors in an integration manufacturing process so that pressure acts upon the pressure sensors when a finger is pressed against the surface of the flexible layer.
The pressure acts only upon the pressure sensors which correspond to ridge portions of the fingerprint, and deflective deformation is produced in the insulating layer extending above the cavity by the action of the pressure.
When piezoelectric resistors are used as the pressure sensors, the resistance values of the piezoelectric resistors varied by the deflective deformation of the insulating layer are utilized so that pressure distribution originating from the concave and convex configuration of the fingerprint is outputted as electric signals. When variable capacitors are used as the pressure sensors, the thickness of the cavity is varied by the deflective deformation of the insulating layer and the variation in electrostatic capacity between two electrodes provided across the cavity is utilized to output electric signals corresponding to the pressure distribution. Further, when micro-conductors are used as the pressure sensors, two electrodes disposed across the cavity are brought into contact to establish a conducting state when the cavity is crushed by the deflective deformation of the insulating layer, so that electric signals corresponding to the pressure distribution are outputted.
Whichever kind or pressure sensor is utilized, in the present conventional example since it is important that the pressure acting on the sensors efficiently produce the deflective deformation of the insulating layer, it is necessary for the flexible layer, that is used as a protective film over the sensor, to have a degree of flexibility so that it does not disturb the deflective deformation of the insulating layer.
On the other hand, if the flexibility of the flexible layer is such that the flexible layer is easily crushed by the pressure acting thereupon, the deflective deformation of the insulating layer will be reduced by the amount of the pressure absorbed in the deformation of the flexible layer itself.
In order to reduce the ratio at which the acting pressure is absorbed by the flexible layer and maintain enough flexibility to not disturb the deflective deformation of the insulating layer, the flexible layer must be made thin. This, however, gives rise to a problem that the surface protecting function which is the original object of the flexible layer is deteriorated significantly.
A pressure sensor which makes use of the fact that the thickness of the cavity is varied by an action of the pressure and this in turn varies the electrostatic capacity is disclosed in Japanese Patent Laid-Open No. 22178/1986 prior to the conventional technique disclosed in Japanese Patent Laid-Open No. 126918/1997 mentioned hereinabove.
FIG. 2 is a sectional view showing a structure of the field effect type pressure sensor disclosed in Japanese Patent Laid-Open No. 22178/1986.
As shown in FIG. 2, in the present conventional example, gate electrode 606 of a field effect transistor and gate insulating film 605 between channels are defined by cavity chamber 607. When the gate capacitance is varied as pressure acts, channel current is modulated and an electric signal output is obtained. Further, in the present conventional example, also a pressure sensor made of a high molecular compound having a superior elasticity is filled in to the cavity.
In the present conventional example, the variation in thickness of the cavity due to pressure is required to be as great as possible. To this end, it is important that the elastic layer made of a high molecular compound have a sufficient flexibility to absorb pressure acting thereupon as much as possible and more preferably is set so as to allow deformation also in directions perpendicular to the thickness.
However, in the present conventional example, since it has the structure that a high molecular compound having a superior elasticity is enclosed in the cavity, the resilient layer cannot be deformed in directions perpendicular to the thickness thereof. As a result, there is a problem that the deformation of the resilient layer in the direction of the thickness by an action of a pressure is limited significantly.
FIG. 3 is a view showing a construction of a pressure type fingerprint inputting apparatus disclosed in Japanese Patent Laid-Open No. 204374/1985, and FIG. 4 is a partial sectional view of a fingerprint inputting plate shown in FIG. 3 which is in an assembled state.
The fingerprint inputting apparatus in the present conventional example includes matrix electrode sheet 720, insulating sheet 721 and pressure sensitive sheet 722 layered one on another as shown in FIG. 3. Matrix electrodes are formed on matrix electrode sheet 720 by forming a plurality of X direction scanning electrodes 1.sub.x1, 1.sub.x2, . . . in parallel by vapor deposition or sputtering on an upper face of substrate 720a made of a material such as alumina or a semiconductor and forming a plurality of Y direction scanning electrodes 1.sub.y1, 1.sub.y2, . . . in parallel to one another but perpendicularly to X direction scanning electrodes 1.sub.x1, 1.sub.x2, . . . by a similar method on a lower face of substrate 720a. Y direction scanning electrodes 1.sub.y1, 1.sub.y2, . . . are partially exposed to the upper face of substrate 720a through through-holes to define conductor portions P.sub.y1, P.sub.y2, . . . .
Insulating sheet 721 has a large number of openings 721a formed therein such that they are positioned at locations centered at intersecting points of the matrix electrodes formed on matrix electrode sheet 720.
Pressure sensitive sheet 722 has a resistance value which varies in accordance with the magnitude of a pressure acting thereupon.
If finger 730 is placed on and pressed against the fingerprint inputting apparatus as seen in FIG. 4, then pressure sensitive sheet 722 is deflected around fulcra provided by a frame of opening 721a and brought into contact with X direction scanning electrode 1.sub.x (for example, X direction scanning electrodes 1.sub.x1) and conductor portion P.sub.y (for example, conductor portions P.sub.y1) of Y direction scanning electrode 1.sub.y (for example, Y direction scanning electrode 1.sub.y1) on the upper face of matrix electrode sheet 720, whereupon the lateral resistance value between them varies in response to the pressing force. Consequently, the variations in resistance value in accordance with the fingerprint pattern can be detected as variations in current value.
FIG. 5 is a view showing a construction of a contact type fingerprint inputting apparatus disclosed in Japanese Patent Laid-Open No. 310087/1988.
As shown in FIG. 5, in the present conventional example, a fingerprint inputting plate is formed on a flat insulating plate against which fingertip 820 is to be pressed, and a contact plate and a matrix circuit unit successively layered on a surface of the insulating plate. The contact plate has spot-like contact electrodes 812 provided thereon at intervals sufficiently smaller than the pitches of ridge portions 820a and valley portions 820b of a fingerprint pattern. The matrix circuit unit has a plurality of scanning electrodes disposed in such a manner as to intersect with each other at the positions of spot-like contact electrodes 812 to form a matrix. Further, detection electrode members are provided individually in a spaced relationship from the contact electrodes such that, when fingertip 820 is placed across the fingerprint inputting plate and the detection electrode members and the scanning electrodes are scanned in a predetermined order, electric conducting and non-conducting states between the contact electrodes and the detection electrode members depending upon whether each of the contact electrodes corresponding to the intersecting points of the scanning electrodes contacts with the fingerprint at ridge portion 820a or does not contact with the fingerprint at valley portion 820b are extracted as fingerprint data.
As described above, in the prior art, an apparatus which satisfies all of reduction in size, reduction in cost and high reliability which are important conditions for practical use is not available as yet.
In particular, in an optical fingerprint detection apparatus which uses a rectangular prism, while optical parts such as a prism and an optical lens are used, optical parts at present have a limitation when it is intended to achieve reduction in size and reduction in cost of an apparatus.
Further, since the principle in detection of a fingerprint pattern makes use of a difference in refractive index of an optical reference surface against which a finger is pressed, the optical fingerprint detection apparatus has a problem in that it cannot achieve high reliability in that the fingerprint pattern is blurred or obliterated by an influence of the contacting condition when the finger is pressed against the optical reference surface and besides by an influence of a dry condition, a wet condition or some other condition of the finger.
Further, also with a fingerprint detection apparatus wherein a layer made of a resilient transparent optical member is formed on a surface of a prism against which a finger is pressed to improve close contact with the finger, the problem of reliability that the fingerprint pattern is liable to be disordered by a dry or wet condition of a finger is not solved.
Further, the fingerprint detection apparatus just described has another problem in that the surface of the layer made of a resilient transparent optical member is more liable to suffer from sticking of soiling matter than a face of glass and a large amount of such sticking matter makes production of a good fingerprint pattern difficult.
Meanwhile, although a fingerprint sensor formed from a semiconductor integrated circuit has been proposed as an effective means for achieving reduction in size and reduction in cost, it does not have sufficient utility in terms of reliability.
In an integrated fingerprint sensor which makes use of the fact that a pressure pattern or a temperature pattern can be obtained from a concave and convex pattern of a fingerprint and outputs electric signals corresponding to a fingerprint pattern through conversion elements disposed in arrays of rows and columns in order to extract variations in pressure or temperature as variations in resistance or voltage, since the protective film such as an oxide film or a nitride film which covers over the surface is hard and thin, it is liable to be damaged, and if even a small crack or pinhole is produced in the protective film, then an impurity such as sodium which sticks to the protective film as a result of contact of a finger with it penetrates through the crack or pinhole and makes a cause of deterioration of a characteristic of the integrated conversion elements or circuit elements.
Meanwhile, in another integrated fingerprint sensor which detects concave and convex patterns of a fingerprint as variations in electrostatic capacity and extracts the variations as electric signals, in addition to the problem described above, if the surface of the protective film is soiled by sweat or the like and the insulation characteristic of the surface is deteriorated, then leak current is produced on the surface and this gives rise to a problem that the contrast of the fingerprint pattern is decreased and, in an extreme case, the fingerprint pattern itself becomes indiscernible.
Furthermore, in a further integrated fingerprint sensor which utilizes a piezoelectric resistor, a variable capacitor or a micro-conductor positioned on an insulating layer extending above a cavity, in order to reduce the ratio at which the pressure acting upon the insulating layer is absorbed by the flexible layer and hold such a flexibility that it does not disturb deflective deformation of the insulating layer, the thickness of the flexible layer must be thin. This gives rise to a problem that the surface protecting function which is the original object of the insulating layer is deteriorated significantly.
Further, with other pressure sensors wherein variations in pressure are detected as variations in electrostatic capacity, since the insulating layer cannot be deformed in directions perpendicular to the direction of the thickness, there is a problem that also deformation of the insulating layer in the direction of the thickness by an action of a pressure occurs less likely.